Methods and apparatus for synchronously combining signals from plural transmitters

ABSTRACT

Enhanced reception of transmitted signals in a communication system is achieved by synchronously combining transmissions from a cluster of transmitters at a distant receiver. The transmitters coordinate transmissions such that each substantially simultaneously transmits the same signal on the same communication channel. As a consequence of the spatial diversity of the transmitters, the transmitted signals arrive at the receiver at different times. The receiver essentially treats the different transmitted signals as though they were different multipath signals from a single transmitter. A multipath equalizer or combiner is used to determine timing offsets among the received signals, and the received signals are time aligned by phase rotating the signals in accordance with the estimated timing offsets. The time-aligned signals are then coherently combined and detected. The combined signal has a greater signal-to-noise ratio than the individual received signals, permitting detection at a greater range or with a lower bit error rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for enhancingreception of transmitted signals and, more particularly, to techniquesfor synchronously combining transmissions from plural transmitters at adistant receiver to extend range performance.

2. Description of the Related Art

One factor affecting range performance in wireless communication systemsis the power with which signals are transmitted. Generally, the strengthof a received signal is proportional to the transmit power and inverselyproportional to the square of the range between the transmitter andreceiver. At a given transmit power, as the range between a transmittingdevice and a receiving device increases, the signal strength at thereceiving device becomes increasingly attenuated, eventually prohibitingreception. Range performance improves with increasing transmit power;conversely, lower transmit power reduces the maximum range at whichtransmitted signals can be detected.

Unfortunately, there are a number of circumstances in which transmitpower is limited by equipment capabilities, operational requirements orboth. For example, with mobile communication devices which rely onbattery power supplies, the maximum transmit power may be limited bydesign to achieve a tradeoff between operating range and battery powerconsumption. Other equipment cost or performance considerations maydictate transmit power capabilities that limit operational range orlimit system performance under harsh operating conditions.

In certain circumstances, a need may exist to minimize RF emissions. Inmilitary contexts, particularly in battlefield situations, minimizingtransmit power reduces the likelihood of signal detection by hostileparties, thereby preventing the transmitter's position from beingcompromised. For example, in a situation where a small team ofradio-equipped personnel is located within a hostile region and mustcommunicate with a distant device, for any single radio to communicatewith the distant device, considerable transmit power would be required,resulting in a significant opportunity for detection by hostile forces.Moreover, the transmit power level required to communicate oversignificant distances would likely drain the radio's battery morerapidly than desired.

In other contexts, low transmit power levels may be advantageous orrequired to minimizing interference with other devices, particularly inhigh bandwidth usage situations, such as with wireless telephony. Ingeneral, it would be advantageous in a variety of applications toachieve certain range performance with reduced transmit power levels or,conversely, to increase range performance without increasing transmitpower.

Operational parameters other than transmit power can be optimized forbetter range performance under certain conditions. For example, antennagain may be increased using a directional antenna, or more sophisticatedreceiver schemes can be employed. In some cases, the problem of limitedrange performance (or, equivalently, limited transmit power) may beovercome using repeaters to boost the signal power at an intermediatelocation between the source transmitter and destination receiver.However, each of these solutions has drawbacks, such as increased size,cost and circuit complexity, overall system complexity, and increasedenergy requirements. Such solutions are especially disadvantageous incovert military situations where minimizing size and transmit power andmaximizing stealth are of utmost importance. Accordingly, it would behighly desirable in power-limited scenarios to enhance range performancewithout resorting to such solutions.

SUMMARY OF THE INVENTION

Therefore, in light of the above, and for other reasons that becomeapparent when the invention is fully described, an object of the presentinvention is to enhance the range performance of a group ofcommunication devices communicating with a distant receiving device,thereby permitting communication over a range greater than thatachievable by any single device transmitting at a particular powerlevel.

Another object of the invention is to reduce the transmit power requiredfrom any single communication device in order to reduce the probabilityof signal interception by unintended recipients.

A further object of the invention is to increase the overall signalstrength of a transmitted signal at a receiver without having toincrease the transmit power from an individual transmitter.

Yet another object of the invention is to coordinate signaltransmissions of plural communication devices to effectively send thesame signals from plural locations to a common receiver device andthereby improve the detectability of the information in these signals.

Still another object of the invention is to take advantage of collectivetransmit power of clusters of communication devices to communicatesignals with greater effective transmit power than is available fromindividual devices in the cluster.

The aforesaid objects are achieved individually and in combination, andit is not intended that the present invention be construed as requiringtwo or more of the objects to be combined unless expressly required bythe claims attached hereto.

Enhanced reception of transmitted signals in a digital communicationsystem is achieved by synchronously combining transmissions from acluster of transmitters at a distant receiver. The transmitterscoordinate their transmissions such that each substantiallysimultaneously transmits the same signal on the same communicationchannel. As a consequence of the spatial diversity of the transmitters,the transmitted signals arrive at the receiver at different times. Thereceiver essentially treats the different transmitted signals as thoughthey were different multipath signals from a single transmitter. Amultipath equalizer or combiner is used to determine timing offsetsamong the received signals, and the received signals are time aligned byphase rotating the signals in accordance with the estimated timingoffsets. The time-aligned signals are then coherently combined anddetected. The combined signal has a greater signal-to-noise ratio thanthe individual received signals, permitting detection at a greater rangeor with a lower bit error rate without having to increase the transmitpower of any individual transmitter. Consequently, enhanced signalreception and range performance can be achieved in systems wheretransmit power is limited by operational constraints or equipmentlimitations.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,particularly when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual representation of the technique of the presentinvention in which a plurality of transmitters communicate with areceiver by synchronously transmitting the same signal.

FIG. 2 is a graph illustrating the coherent integration loss due tophase errors among received signals.

FIG. 3 is a graph illustrating achievable communications performance ofan exemplary embodiment of the present invention on a Rayleigh fadingchannel for PSK signaling.

FIG. 4 is a graph illustrating achievable communications performance ofan exemplary embodiment of the present invention on a Rayleigh fadingchannel for DPSK signaling.

FIG. 5 is a graph depicting the coverage characteristics for tensynchronized transmitters in terms of coherent integration gainaccording to one embodiment of the present invention.

FIG. 6 is a schematic block diagram of a synchronous combining receiveraccording to an exemplary embodiment of the present invention.

FIG. 7 is a more detailed circuit diagram of the differential detectiontap delay and rake tap selection processing of the synchronous combiningreceiver according to the exemplary embodiment of the present invention.

FIGS. 8A-8D illustrate formats for RTS/CTS/ACK data packets used forCSMA/CA message transmission in accordance with an exemplary embodimentof the present invention.

FIGS. 9A-9D illustrate formats for TRANSEC data packet communication inaccordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves a signal combining technique in whichsignals are transmitted in a coordinated manner from pluralcommunication devices in relatively close proximity to each other (e.g.,a “cluster” of devices) to a receiving communication device whichcoherently combines the signals as if they were different multipath raysof a single transmission. The combined signal power enables reception ofsignals over ranges far greater than would otherwise be possible with asingle device transmitting at the same single-device power level.

The signal combining concept of the present invention uses thecollective resources of a number of communication devices bysynchronizing or coordinating transmission and reception of signals.Proper coordination enables the cluster of communication devices tocollectively transmit the same information-bearing signals, such that anintended receiver can process the received signals to significantlyimprove the communications performance (e.g. quality of service, range,etc.) with efficient utilization of transmitting energy.

A representation of the concept underlying the present invention isillustrated in FIG. 1. Each of a plurality of M communication devices islocated within a limited geographical area, such that each of thecommunication devices can directly communicate with at least one of theother M communication devices, thereby enabling radio communicationamong the communication devices. A group of communication devices soarranged can be considered to be in a “cluster.” As used herein and inthe claims, the term “cluster” refers to a set of two or morecommunication devices so position to permit a coordinated transmissionof combinable signals from the communication devices to a commonreceiver. While the term cluster generally implies some degree ofrelatively close proximity, the present invention is not limited to anyparticular maximum distance between devices in the cluster or maximumouter boundary for the cluster. Preferably, although not strictlyrequired, all of the M communication devices are within the field ofview or line of sight of each other, such that any two of the devicescan communication with each other directly. Some of the transmitters mayalso be in motion relative to the other transmitters and to thereceiver.

As used herein and in the claims, a “communication device” includes anydevice, mobile or stationary, that is capable of transmitting and/orreceiving communication signals, including but not limited to: ahandheld or body-mounted radio; any type of mobile or wireless telephone(e.g., analog cellular, digital cellular, PCS or satellite-based); apager, beeper or PDA device; a radio carried on, built into or embeddedin a ground-based or airborne vehicle; any portable electronic deviceequipped with wireless reception/transmission capabilities, includingmultimedia terminals capable of receiving/transmitting audio, videoand/or data information; and any device mounted in a fixed location withtransmission/reception capabilities.

While the signals transmitted from the different transmittingcommunication devices arrive at the receiving communication device atdifferent times, it should be understood that the signals arenevertheless transmitted on the same communication channel in much thesame manner as multipath signals from a single transmitter are on thesame communication channel. Thus, for example, if the system employscode division multiple access (CDMA), all of the transmitted signals arespread using the same code (e.g., the same PN code, Walsh function,etc.). Likewise, if the system employs frequency division multipleaccess (FDMA), all of the transmitted signals are on the same frequencychannel within the available frequency band.

At least one receiving communication device is remotely located from thecluster of transmitting communication devices. To be useful intransmitting signals to a particular receiver, each of the Mcommunication devices must be positioned such that its transmittedsignals can travel over a path to the receiver and contribute to thepower of the combined received signal. As shown in FIG. 1, the receivermay be located at a considerable distance from the communication devicesin the cluster. In fact, an important aspect of the present invention isthe ability to receive signals from the cluster of communication devicesbeyond the maximum reception range possible with an individualcommunication device transmitting at a specified power level. Due to thedistance between the cluster of transmitters and the receiver, thesignal quality of the communication between individual transmitters andthe receiver may be poor. However, by synchronously combining each ofthe transmitted signals at the receiver as multipath rays correspondingto a single signal, the gain of the received signal may be significantlyincreased.

While the advantages of the present invention are readily apparent inscenarios where the receiver is located relatively far from the clusterof transmitters, it should be understood that the present invention isnot limited to any particular receiver location or any particularminimum range from the cluster. For example, the signal combiningtechnique of the present invention can be implemented such that when thereceiving communication device is well within the operating range of thetransmitting devices, the transmit power of each of the transmittingdevices can be reduced accordingly.

The communication devices in the cluster transmit the same signal to thereceiving device and coordinate their respective transmit times suchthat the signals arrive at the receiver within a narrow time window thatallows the receiving device to constructively combine the pluralsignals. One of the transmitting communication devices is identified asthe lead communication device. The lead device communicates with otherdevices in the cluster to coordinate transmission of a signal to thereceiving device. Preferably, the lead communication device is thedevice initiating transmission (i.e., the device whose operator desiresto transmit a message to the receiving communication device).

The lead device can be any of the devices in the cluster. For example,consider the case where the communication devices are in a peer-to-peernetwork. A key design premise of a peer-to-peer network is that thereare no pre-determined “lead” devices. Following this fundamentalprinciple, any transmitter wanting to initiate a synchronizedtransmission will communicate this intention to neighboring devices viaspecial message. The initiating device does not need to know where theother members of the network are, nor how many devices there are toreceive the message. The cluster of transmitting devices then operatesto simultaneously transmit a data communication signal comprising aknown data sequence portion or acquisition/synchronization portion andan information bearing signal portion containing the communication ofinterest to be transmitted to the receiving device. Note that eachtransmitter operates to transmit the same information bearing signalportion and also transmit the known data sequence portion or a serialprobe portion.

In accordance with an exemplary embodiment, the transmittingcommunication devices attempt to transmit their respective measurementsat the same instant in order to minimize the span of time over which thesignals arrive at the receiver. Of course, due to synchronization errorsand variations in processing times, the actual transmit times of thesignals from the respective transmitting communication devices may varyslightly. More generally, the signal transmit times of the transmittingdevices may differ to a limited extent, either by design or due tosystem timing errors, so long as the arrival times of the signals at thereceiving device fall within a time window that enables the receivingdevice to combine the signals. Accordingly, the phrase “substantiallysimultaneously” as used herein and in the claims in connection withtransmitting signals means that the signals are transmitted close enoughin time to be received by the receiver within a time window that permitsthe signals to be combined.

In accordance with the exemplary embodiment, the overall system issynchronized relative to the Time-of-Day (ToD). Initially, the firstcommunication device in the cluster which is turned on, R₁, will receivea signal indicating ToD to a resolution of 1 nsec (this is resolutionnot accuracy), for which 64 bits are sufficient. Specifically, for aone-hundred-year interval,D=100*365*24*60*60*1000*1000*1000=3.154×10¹⁸Nbits=log 10(D)/log 10(2)=61.5

Communication device R₁ initiates the clock counter and broadcasts a ToDmessage, M_(ToD), to all other communication devices in the clusternetwork. The network communication devices R₂ . . . R_(N) will receiveM_(ToD) and set their own clock counters relative to the ToD containedin the message, thereby establishing a common time reference among thedevices.

The radius which covers all the network radius is related to themultipath window as follows. A radiowave propagates at a velocity of3.333 10⁻⁶ sec/km (3.333 μsec/km). This means that, in a network havinga cluster of devices spread over a radius of 1 km, the ToD will beoffset by at most 3.333 μsec. This coarse synchronization is sufficientfor the system of the present invention to work correctly, provided thatthe multipath window at the receiver includes the propagation delaysnecessary to receive all transmitters. For example, a multipath windowof ±25 μsec will readily be able to receive and combine all signalstransmitted from the cluster, as described in greater detailhereinbelow.

To effect a transmission, one of the communication devices R_(i)distributes a message M_(data) to N neighboring communication devicesthat will be required to transmit in synchronism. This message containsa specified future ToD, T₀ when the transmission will occur as well asthe destination address of the receiving device, designated R_(K).Observe that T₀ can be any time beyond the maximum propagation delay forthe network radius. At the specified time T₀, the N radios transmit thespecified message to a sole destination R_(K). The receiver at R_(K)will receive all transmissions with maximum offsets given by theinitialization propagation delay, d_(initial), plus the messagetransmission propagation delay, d_(tx), plus the local clock time drift,L_(drift). With a local clock (oscillator) accurate to ±1 part permillion (10⁻⁶), the total time offset will be:T _(offset)=3.333 μsec+3.333 μsec+1 μsec=7.666 μsec,which is well within the ±25 μsec multipath window stated above. Thismeans that the receiver will appropriately combine the signals as thoughthey were multiple reflections of an original signal. Importantly, noprecise phase synchronization is required at the transmitters, since allthat is required is that the set of transmitted signals arrive withinthe multipath window at the receiver. Like multipath components, theseveral signals transmitted by the different transmitters in the presentinvention arrive at the receiver with completely random phases. It isthe function of the multipath combiner at the receiver to align thephases of the arriving signals. It should be understood that theinvention is not limited to the specific values used in the foregoingexample. In general, the system will operate correctly if the multipathwindow is set to cover twice the maximum propagation delay plus thelocal clock offset. For any specific system, this would be a relationbetween the maximum distance and how accurate (and expensive) the localoscillators are made.

Any suitable messaging scheme can be employed by the cluster oftransmitting communication devices, provided the scheme permits the leadcommunication device to arrange the substantially simultaneoustransmission of the message from the group of transmitting devices. Byway of non-limiting example, the lead communication device can send aspecial Request to Send (RTS) message to the neighboring communicationdevices. When receiving the special Clear to Send (CTS) from theneighboring devices, the lead device then initiates transmission withina given delay time. The time delay can synchronized by Time of Day anddirectly derived from the Key Generator Transition switching point, i.e.the Epoch. This is the point where the KG is reloaded to generate newnon-linear spreading sequences. The time interval, or Epoch, depends onthe particular system, and for reference purposes can be set initiallyto 1 sec.

According to another approach, the device whose position is determinedto be closest to the receiving device may be designated as the leaddevice. In this case, the transmitting devices communicate with oneanother to obtain a precise geographic location of one of thetransmitting devices, such as the transmitting device closest to thereceiving device or the lead device. Such a determination may be madeusing a variety of methods including use of global positioning system(GPS) data for determining precise geometric coordinates. It should beunderstood, however, that in general, position information is notnecessary for the correct functioning of the system of the invention.Insofar as the time delays from each transmitting station are within themultipath window set by the receivers, the system will work correctly.

As previously noted, because of the distributed locations of thetransmitters, the simultaneous transmissions from the cluster oftransmitters arrive at the receiver at different time offsets relativeto one another. In accordance with the present invention, at thereceiver, the signals transmitted from the individual communicationdevices in the cluster are essentially treated as though they aredifferent multipath rays of a single transmission, and multipathcountermeasures are used to process and combine the receivedtransmissions from the different communication devices. As will beexplained in greater detail hereinbelow, this task is accomplished usinga multipath combiner device, such as an equalizer, with the optionaladdition of a serial probe.

A brief explanation of the phenomenon of multipath fading will aid inunderstanding operation of the receiver of the present invention. Interrestrial-based radiowave propagation, multipath interference occurswhen reflected rays originating from a radio transmitter arrive at thereceiver delayed in time by τ_(m) relative to the arrival time of thedirect-path ray. The delayed signals will then combine constructively ordestructively with the direct-path ray and with each other, depending ontheir relative phases at the receiver antenna, giving rise to thephenomenon called multipath fading. A simple baseband equivalent modelcan illustrate the problem. A received signal is given by:r(t)=Σa(t)exp(−j2πf _(c)τ_(n)(t))  (1)where the received signal is the sum of a number of time variant vectors(phasors) having amplitudes a(t) and phases θ=2πf_(c)τ_(n)(t). Note thata change in amplitude requires a large change in a(t), while θ willchange by 2π every time that τ changes by 1/f_(c). For example, atfrequency f_(c)=2,400 MHz, 1/f_(c)=0.5×10⁻⁶ seconds. Since radio wavestravel at approximately one ft/nsec, a path delay at higher frequenciesdoes cause significant changes in the phase of the signal, causing thesignal distortion known as multipath fading.

This is further complicated by motion. If the transmitter and receiverare moving with respect to each other, then another key aspect of themultipath fading is the fade rate, which is a function of the relativevelocity between the transmitter and the receiver location at points Aand B, where A and B are separated by the distance traveled by thereceiver at velocity v. The difference in path lengths traveled by awave due to the movement of the receiver, measured in wavelengths, λ,yields a phase difference ΔΦ=2πΔ1/λ, where Δ1=v Δt Cos(θ), where θ isthe angle between transmitter/receiver path and the direction ofmovement. The waves will be exactly at opposite phases at a rate givenby the Doppler shift:f _(d)=(1/2π)(ΔΦ/Δt)cos(θ)=(v/λ)cos(θ)  (2)Consequently, the signal will have deep fades at the Doppler rate.

Doppler Spread is defined as the frequency interval f_(c)−f_(d),f_(c)+f_(d). Coherence Time (T_(c)) and Doppler spread characterize thetime varying nature of the channel, caused by the relative motion oftransmitter and receiver and by the movement of reflective objects inthe channel. The coherence time T_(c), gives the interval of time overwhich the fading characteristics will not change, i.e., an equalizerwill have to be re-trained at T_(c) intervals of time. For a velocity of500 mph, T_(c)=229×10⁻⁶ seconds.

As will be appreciated from the foregoing explanation of multipathfading, an analysis of a signal-combining system is quite complex foroperations in restrictive environments, because the nature of multipathcharacteristics varies with time. As a result, the transfer function ofthe transmission medium is time varying, which may be characterizedstatistically in terms of the coherence bandwidth and time of thechannel. To assure analytical tractability, the mathematical model for asignal combining system is described based on the concept of diversity.It is assumed that there are L diversity channels, each carrying thesame information-bearing signal. The fading processes among the Ldiversity channels are assumed to be statistically independent.

The system according to the present invention employs a multipathcombiner or equalizer, often called a “rake” receiver. A rake receivergenerally includes parallel correlator circuits which receive signalsincoming from different signal paths. In code division multiple access(CDMA) systems, for example, several components are resolved at aresolution equal to the chip period and are coherently combined.Coherent combining of the signals requires that the signal havesubstantially the same phase and frequency. Thus, in multipath fadingenvironments, instead of losing a signal by destructive interference ofmultipath components, two or more different path signals are received,and phase adjustment is made to provide constructive combining of thesesignals.

In accordance with an exemplary embodiment of the present invention, therake receiver employs a tapped delay line through which the receivedsignal is passed. The signal at each tap is properly weighted andsummed, in effect, to collect the signal energy from all the receivedsignal paths that fall within the span of the tapped delay line andcarry the same information. FIG. 2 shows the coherent integration lossdue to phase errors among the received signals. Note that significantgains can be realized as long as phase errors among the received signalscan be maintained within 90 degrees. FIGS. 3 and 4 illustrate thecommunications performance of the signal combining technique of thepresent invention on a Rayleigh fading channel for PSK and DPSKsignaling, respectively. The communications performance is shown interms of the probability of a bit error (BER) as a function of therequired signal-to-noise (SNR) per bit in dB. The required SNR per bit,denoted by R_(b), is L times the SNR per bit per channel, denoted byR_(c). L indicates the order of diversity. For a given BER, the requiredR_(b) can be determined, and then the required R_(c) for eachtransmitting channel can be estimated. With the order of diversity, theperformance gain of the signal combining system of the present inventionover a single radio system can easily be estimated. FIG. 5 depicts thecoverage characteristics for ten synchronized transmitters in terms ofcoherent integration gain.

A schematic block diagram of the receiver system according to a presentinvention is shown in FIG. 6. A front end of the receiver (not shown) iscoupled to an input terminal 5. The front end of the receiver mayinclude a tuner and intermediate frequency (IF) stage, ananalog-to-digital converter, and a 90 degree phase shifter coupledtogether in a known manner to produce various clock signals required bycircuitry in the receiver in synchronism with the received signal. Thetiming reference generator and clock recovery circuit may also belocated after the multipath equalizer.

The receiver front end digitizes the IF signal to produce and in phase(I) component and then rotates the digital signal to obtain thequadrature (Q) component, and the digital I and Q components arereceived at the input terminal 5. The I and Q signals are supplied to adigital matched filter 50 whose output is supplied to the multipathequalizer or combiner comprising a plurality of tap delay lines 40 forreceiving each of the phased shifted data communication signals. Eachtap delay line has corresponding weighting coefficients h[0], h[1], . .. h[m] associated with each of the received channels for adjusting theamplitude and phase of the received signal in order to equalize thesignals. Each of the tap delay lines is coupled to a correlator 55 forproviding a correlation signal indicative of the amount of correlationamong the received time delayed signals. The peaks of the correlationsignal are detected in a detector 60 which is coupled to phase rotator64 for rotating the phase of the detected peak signal to allow forcoherent combination. Detector 60 detects the N highest peaks receivedfrom correlator 55. In general, any practical number of peaks N whichexceeds a given threshold can be detected up to the total number ofpeaks. Unless limited by hardware or processing considerations, it ispreferable to select and process all peaks exceeding a given threshold;however, in certain circumstances it may be preferable to specify apredetermined maximum number of peaks to be processed.

The phase-rotated signals produced by phase rotator 64 are received by acombiner 70 which coherently combines each of the detected peak signalsto produce a combined output 80. Phase rotator 64 essentially timealigns the separate signals by ensuring that the relative phases of thesignals are within a certain number of degrees of each other, such thata significant integration gain will result when the signals arecoherently combined. A threshold detector 90 compares the combinedsignal 80 with a predetermined threshold to determine whether or not atrue signal is present. The combined detected signal has a gainexceeding that of each of the individual data communication signals, andis indicative of the data communication signal.

The rake receiver operates on each of the received data communicationsignals from the simultaneous transmission of the transmitting devicesas if the transmissions were multipath signals from a particular source.The present invention utilizes such information to enhance the gain bycombining these signals in synchronous fashion. In a practicalimplementation, important aspects of the system include the use ofDirect Sequence Spread Spectrum (DSSS) and a Digital Matched Filter(DMF). The DSSS expands the data pulse bandwidth with a secondarymodulation called chipping. For example, with a 1 MHz (Megabits persecond) signal spread with a 32 MHz chipping signal the symbol durationis T_(s)=1 μsec, and a chip duration T_(c)=31.25 nsec. Consequently,each stage in the DMF is 31.25 nsec apart, such that different signalpaths separated by at least 31.25 nsec will be recognized.

Another important aspect of the invention is that the signal is sampledat the Nyquist rate (two times the chipping rate, or 64 MHz), such thatthe system produces one output of the DMF at the sampling rate, i.e. 64MHz, meaning that each 15.625 nsec there will be an output. Over aperiod of 1 μsec, there are 64 outputs and, over a period of 0.250 μsec,there are 256 outputs, which is consistent with the synchronizationspeed of practical systems. Theoretically, all these outputs can becombined with a Digital Filter (Finite Impulse Response). This FIR wouldthen be an “equalizer”, since having a definite mathematicalrelationship between the input and the output impulse responses of anequalizer FIR makes it indeed an equalizer. However, the presentinvention does not require a true equalizer. In accordance with anotherimplementation, heuristic “combining” can be used, i.e., phase matchingand adding the magnitude of selected paths. The combining approach canbe advantageous where hardware limitations or costs are a consideration.The particular combining/equalization method is not critical to theinvention. An equalizer combiner will produce better results than anon-equalizer combiner; however, within a given signal to noise ratiolimitations both approaches work.

The serial probe provides an instantaneous measure of the channelimpulse response at the coherence time intervals. Essentially, theserial probe involves a sequence of known pulses which can be identicalto the synchronization sequence which can be used to determine how thesignal was distorted in the channel. The channel impulse responseaccounts for the relative motion, not only between the receiver and thetransmitter, but the relative motion between the transmitters. Theserial probe provides information to set the RAKE combiner tap weights(this can be understood as the coefficients in a Finite Impulse Response(FIR) filter). This minimizes the losses due to channel variability aswell as frequency offsets due to Doppler shifts. The serial probe is notstrictly necessary for the system of the present invention to operatecorrectly; however, performance is degraded without the serial probe.

The serial probe is preferably identical to the synchronization sequencein the transmitted message, for example, sixteen words of 4 μsecduration each. The chip pattern changes according to the KG Epoch;however, the underlying symbol pattern can remain the same or changewith the Epoch as required for different missions. The symbol pattern isan M-Sequence generated by a Linear Sequence Generator, e.g., 16, 12, 3,1.

The serial probe is inserted at intervals determined by the CoherenceTime. This is the time over which the channel impulse response isinvariant, and it is dependent on frequency given by:$T_{c} = \sqrt{\frac{9}{16\pi\quad f_{m}^{2}}}$

The serial probe is processed at these intervals and all the subsequentsymbol inputs are processed with the information (tap weights) derivedfrom the last probe. The serial probe is also used to resynchronize thesystem, i.e., the peak correlation output of the probe sets the initialtime to for the subsequent symbols.

According to an exemplary embodiment of the present invention, the rakereceiver may be embodied within a digital radio. For example, such asystem may comprises a burst packet local area network having a layeredarchitecture, where communications are established at the applicationslevel and remain until one of the participants terminate it. Such atransmitter and receiver system may utilize a frequency or code divisionmultiple access communications scheme (or both) comprising a combinationof channel sense multiple access/collision avoidance (CSMA/CA) and codedivision multiple access (CDMA) techniques for enhancing systemthroughput.

In this exemplary embodiment, by way of non-limiting example, four 20MHz channels are available within a 2400-2800 MHz frequency band andseveral signals may be superimposed within the same time bandwidth,through the use of appropriate spreading codes. Note that, in thetransmitter and receiver system described above, signaling comprises twodifferent modulations; data modulation and spreading modulation. A modemoperable within the transmitter/receiver system spreads the basebandsignal (not the up converter frequency) with a quadrature modulation,such that the bandwidth can be contained within 20 MHz. The datamodulation comprises a binary phase shift key (BPSK) sequence, while thespread modulation is an offset quadrature phase shift key (OQPSK) having16 megachips per second (Mcps) for the in-phase component and 16 Mcpsfor the quadrature component.

FIG. 7 is a more detailed block diagram of the rake receiver componentsillustrating differential detection tap delay and the rake tap selectionprocessing implemented within the device of the exemplary embodiment foracquiring and estimating signal channels to adaptively control networkcommunications for enhancing the gain of received signals. Referring toFIG. 7, in phase (I) and quadrature (Q) channel signals are supplied todigital matched filter 15 to provide pulse shaping of the input signal.Output signal 20 from filter 15 comprises an 8 bit complex data sequencewhich is delayed (module 25) and the complex conjugate obtained viamodule 30. The complex conjugate output is then multiplied (module 35)with signal 20 and the real part 45 of the signal is produced by module38. Signal 45 is received by a quantizer 50 which quantizes the signalto 5 bit values, and an 896 chip (32 Mcps) tapped delay line 52 usingweighted taps operates to perform the multipath equalization. Rake tapselection processor 54 is responsive to the output of tapped delay line55 for performing threshold detection and acquisition.

Any of a variety of transmission protocols or schemes can be usedtransmit signals between the cluster of communication devices and theremote communication device receiving the combined signals. By way ofnon-limiting example, the communication devices can communicate witheach another via packet transmission using a carrier sense multipleaccess with collision avoidance (CSMA/CA) scheme. With CSMA/CA, eachpacket transmission between two communication devices typically involvesan exchange of four short bursts. First, a request to send message (RTS)is sent from the sending device to the receiving device. The receiverdevice then responds upon receipt of the RTS message with aclear-to-send message (CTS). A data message (MSG) is then transmittedfrom the sending device, and an acknowledgment message (ACK) is sentfrom the receiving device upon reception of the data message. If thedata message is not successfully received, the receiving device can senta no acknowledgment (NAK) message back to the sending device or simplynot send any message within a time out period, indicating to the sendingdevice that the data message was not successfully received.

FIGS. 8A-8D illustrate exemplary formats for the RTS/CTS/ACK datapackets used for message transmission. The RTS/CTS/ACK acquisitionsequences comprise ten 4 μsec. symbols as shown in FIG. 8A. Referring toFIG. 8B, acquisition data portion 20 is approximately 40 μsec. induration, while the RTS/CTS/ACK message portion 30 is approximately 128μsec. A power amplifier (PA) and automatic gain control (AGC) headerportion 10 of 12 μsec. in duration provides gain control information atthe beginning of the message, while PA tail portion 40 having a 4 μsec.duration provides end of data information (amplifier rise and settletimes). The RTS/CTS/ACK message portion 30 is provided in greater detailin FIG. 8D. FIG. 8D shows the 32 bit sequence where each bit element hasa duration of 4 μsec. The acquisition data sequence 20 is furtherillustrated in FIG. 8C, which shows a 10 symbol sequence having atemporal duration of 4 μsec per symbol.

In a DSSS system where each symbol is spread with different spreadingsequences, the multipath window can cover several symbols. If thesymbols were spread with the same sequences, then multipath rays whichare delayed more than one symbol cannot be combined, since this wouldpresent the risk of combining two different symbols (virtuallydestroying them). In the system of the present invention, with differentspreading sequences for each symbol, a reflected component of symbolS_(K) which will produce a correlation peak only with reference R_(K).Thus, the reference is kept for as long as the desired length of themultipath window.

As can be seen from the diagram of FIG. 8A, the first bit or symbol b1within the sequence represents a reference bit, while bits two and threeare indicative of the message type (i.e. RTS, CTS, or ACK). The nextseven bits (b4-b10) indicate the destination address (for RTS) or sourceaddress (for CTS and ACK) messages. The RTS message further includes themessage data rate (bits b11, b12) message length (b13-15), suggestedmessage channel (b16-18), and noise control bits (b19-20). A 5 bitpacket priority and 7 bit source ID are then appended to the RTS messagespreading sequence.

In similar fashion, the CTS message portion further includes a messagedata rate (2 bits), message length (3 bits), message channel direction(2 bits), power control portion (3 bits), FEC portion (2 bits) and adestination ID (7 bits). Three spare bits (b23-26) are also includedwithin the CTS message type. Finally, the ACK message includes anindicating of the link quality (3 bits, b11-13) and a 12 bit networkprocessor writable and spare capacity segment, in addition to the 7 bitdestination ID.

Thus, FIGS., 8A-8D illustrate the communication protocol used amongtransmitter and receiver devices for communicating over 16 MHz signalbandwidths (20 MHz including the guard band). Note that thecommunication process identified above terminates a transmission betweenthe receiver and transmitter, even though the two terminals are stillconnected at the application level. In this manner, each packet istransmitted independently of all the other packets, so that there is nosignal in the air that indicates that the receiver and the transmitterare connected in any way. This advantageous feature minimizes thepotential for eavesdroppers to detect and/or identify the location ofthe transmitting and receiving devices. Note however, that the packetsdo include sequencing information to allow the receiver to reconstructthe entire message.

For TRANSEC communications, FIGS. 9A-9D illustrate the format for thenetwork packets for RTS/CTS/ACK message transmission using aconvolutional code rate of ½ and K=7 error correcting code. FIG. 9Aprovides the overall TRANSEC format, while FIG. 9B illustrates specificmessage segment formats including the acquisition portion, tap trainportion, data portion, and probe portions of the message. Decisiondirected tap training portion, as shown in FIGS. 9B and 9C, represents a16 symbol field of variable duration ranging between 0.5 μsec. (2 Msps),1 μsec. (1 Msps), 2 μsec. (0.5 Msps) and 4 μsecs. (0.25 Msps).

As shown in FIG. 9B, the data probe portion 35 comprises a known bitpattern which is inserted by the modem into the signal at selected timeintervals derived from propagation theory, and which provides aninstantaneous measure of the channel impulse response in the environmentin which the unit is operating in. The communication device's modemprobes the channel on a period of about 75% of the worst case channelcoherence time. Note that data symbols time sequenced just prior toprobe portion N+1 still use the channel identified by probe N to receivethe signal waveform. The modem uses decision directed feedback to trackchanges in the channel phase, but does not attempt to locate newmultipath waves prior to the next data probe portion. Thus, even thoughthe total energy in the channel is held constant, some energy may movefrom the taps currently being received into one or more of theunmonitored time delays.

Having combined and detected the signals from plural transmittingdevices in the cluster, the receiving communication device may thenreply to the message as necessary. Depending on the power restrictionson the receiving communication device, the reply message may be sentback to the originating device via a single transmission at a higherpower level, or, where the receiving communication device is itselfamong a cluster of devices under similar power restrictions (e.g., asecond convert squad located at a distance from the first covert squad),the same signal combining technique can be used to send signals back tothe originating device.

It should be understood that the present invention is not limited to anyparticular protocol, messaging scheme or type of channel access, and isuseful in any context or network that would benefit from an increase inthe power of received signals resulting from the combined power ofdistributed transmitters. For example, consider a network ofcommunication devices which transmit with very low power due to cost,power and/or RF emissions considerations. While many if not all of thecommunication devices in the network may be within each other's fieldsof view, the power limitations may make direct communications betweencertain devices impossible. One conventional solution to this problemwould be relay messages using intermediate devices in the network. Inaccordance with the present invention, another approach is for thedevice from which the message originates to command devices within itsoperating range to simultaneously transmit signals containing the samemessage, such that the intended receiving device, which may not bewithin the operating range of the originating device alone, isnevertheless within the extended reception range resulting fromcombining the signals of the group of transmitters. In this manner, thetechnique of the present invention may avoid delivering the signal viamultiple “hops” (i.e., through multiple intermediate devices), and maypotentially simplify signal routing algorithms and reduce the need forrouting tables in certain types of networks.

The signal combining technique of the present invention hasapplicability in the context of military or covert field operations. Forexample, a squad of soldiers or a reconnaissance team equipped withradios may be required to transmit using very low power levels tominimize the risk of being discovered by hostile forces or revealingtheir location. At such power levels, reliable communications may bedifficult to achieve between individual radios in the squad and areceiver at a command center located at a considerable distant from thesquad. In accordance with the present invention, range performance canbe enhanced in this scenario without increasing the transmit power ofindividual radios in the squad by substantially simultaneouslytransmitting the same signals from some or all of the radios in thesquad and combining the signals at the distant receiver.

Although the communication devices of the present invention havedescribed herein as being mobile devices, the invention encompassessystems in which some or all of the devices are stationary. For example,the present invention may be useful in networks deployed with officebuildings, hotels, parking garages and/or malls, wherein certaincommunication devices have fixed positions.

Having described preferred embodiments of a new and improved methods andapparatus for synchronously combining signals from plural transmitters,it is believed that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims. Although specificterms are employed herein, they are used in a generic and descriptivesense only and not for purposes of limitation.

1. In a communication system comprising at least a plurality oftransmitting communication devices and a receiving communication device,a method of combining signals respectively transmitted from thetransmitting communication devices to enhance reception at the receivingcommunication device, comprising: (a) coordinating transmissions of theplurality of the transmitting communication devices such that aplurality of transmitted signals are respectively transmitted over thesame communication channel by the plurality of transmittingcommunication devices substantially simultaneously, each of thetransmitted signals including an information signal which is coherentlycombinable with corresponding information signals in others of thetransmitted signals; (b) receiving the transmitted signals at thereceiving communication device such that respective arrival times of thetransmitted signals are offset from one another as a function ofrespective positions of the transmitting communication devices; (c) timealigning the transmitted signals to compensate for the respectivearrival times of the transmitted signals; (d) combining the transmittedsignals to form a combined signal including at least a combinedinformation signal; and (e) detecting the combined signal to determinethe presence of the transmitted signals.
 2. The method of claim 1,wherein: each of the transmitted signals further comprises anacquisition signal; (b) includes correlating the acquisition signal oftransmitted signals received by the receiving communication device to astored signal to estimate the arrival times of the transmitted signals;and (d) includes combining information signals from at least some of thetransmitted signals correlated in (b) to form the combined informationsignal.
 3. The method of claim 2, wherein the acquisition signal in eachof the transmitted signals is identical.
 4. The method of claim 1,wherein (c) includes phase rotating at least some of the transmittedsignals correlated in (b) to adjust a relative timing of the transmittedsignals to account for timing offsets among the respective arrival timesof the transmitted signals.
 5. The method of claim 1, wherein thetransmitted signals arrive at the receiving communication device withinan acquisition time interval having a duration sufficiently short topermit combining of the transmitted signals.
 6. The method of claim 1,wherein the information signal in each of the transmitted signals isidentical.
 7. The method of claim 1, wherein each of the transmittedsignals includes a serial probe comprising a known data sequence, themethod further comprising: (f) determining a channel impulse responsefrom the serial probe.
 8. The method of claim 1, wherein the transmittedsignals are heuristic combined by phase matching and adding themagnitude of the transmitted signals.
 9. The method of claim 1, whereinthe transmitted signals are combined using an equalizer.
 10. The methodof claim 1, wherein at least one of the transmitting communicationdevices is a mobile communication device
 11. The method of claim 1,wherein the receiving communication device is a mobile communicationdevice.
 12. In a communication system comprising at least a plurality oftransmitting communication devices and a receiving communication device,a method of coordinating transmission of signals respectivelytransmitted from the transmitting communication devices to the receivingdevice, comprising: (a) establishing a common time reference among thetransmitting communication devices; (b) commanding the transmittingcommunication devices to transmit signals to the receiving device at afuture time; and (c) respectively transmitting a plurality of signalsfrom the transmitting communication devices at the future time, suchthat the plurality of signals are respectively transmitted over the samecommunication channel substantially simultaneously, each of the signalsincluding an information signal which is coherently combinable withcorresponding information signals in others of the signals.
 13. Themethod of claim 12, wherein one of the transmitting communicationdevices commands others of the transmitting communication devices totransmit signals at the future time.
 14. The method of claim 13, whereinsaid one of the transmitting communication devices broadcasts a commandto said others of the transmitting communication devices.
 15. The methodof claim 12, wherein a time between commanding of the transmittingcommunication devices and the future time is greater than a longestsignal propagation time between transmitting communication devices 16.The method of claim 12, wherein at least one of the transmittingcommunication devices is a mobile communication device.
 17. The methodof claim 12, wherein the receiving communication device is a mobilecommunication device.
 18. The method of claim 12, wherein the commontime reference is the time of day.
 19. A method of detecting a pluralityof signals respectively transmitted substantially simultaneously from aplurality of transmitting communication devices over the samecommunication channel, comprising: (a) receiving the transmitted signalsat a receiving communication device such that respective arrival timesof the transmitted signals are offset from one another as a function ofrespective positions of the transmitting communication devices, each ofthe transmitted signals including an information signal which iscoherently combinable with corresponding information signals in othersof the transmitted signals; (b) time aligning the transmitted signals tocompensate for the respective arrival times of the transmitted signals;(c) combining the transmitted signals to form a combined signalincluding at least a combined information signal; and (d) detecting thecombined signal to determine the presence of the transmitted signals 20.The method of claim 19, wherein: each of the transmitted signals furthercomprises an acquisition signal; (a) includes correlating theacquisition signal of transmitted signals received by the receivingcommunication device to a stored signal to estimate the arrival times ofthe transmitted signals; and (c) includes combining information signalsfrom at least some of the transmitted signals correlated in (a) to formthe combined information signal.
 21. The method of claim 19, wherein (b)includes phase rotating at least some of the transmitted signalscorrelated in (a) to adjust a relative timing of the transmitted signalsto account for timing offsets among the respective arrival times of thetransmitted signals.
 22. The method of claim 19, wherein at least one ofthe transmitting communication devices is a mobile communication device.23. The method of claim 19, wherein the receiving communication deviceis a mobile communication device.
 24. A communication system,comprising: a plurality of transmitting communication devices configuredto respectively transmit a plurality of transmitted signals over thesame communication channel substantially simultaneously, each of thetransmitted signals including an information signal which is coherentlycombinable with corresponding information signals in others of thetransmitted signals; and a receiving communication device configured toreceive the transmitted signals at respective arrival times which areoffset from one another as a function of respective positions of thetransmitting communication devices, said receiving communication devicetime aligning the transmitted signals to compensate for the respectivearrival times and combining the transmitted signals to form a combinedsignal.
 25. The system of claim 24, wherein each of the transmittedsignals further comprises an acquisition signal and said receivingcommunication device comprises: a correlator configured to correlate theacquisition signal of transmitted signals to a stored signal to estimatethe respective arrival times of the transmitted signals; a phase rotatorconfigured to phase rotate at least some of the transmitted signals toadjust a relative timing of the transmitted signals to account fortiming offsets among the respective arrival times of the transmittedsignals; and a signal combiner configured to combine the transmittedsignals to form the combined signal.
 26. The system of claim 25, whereinthe information signal in each of the transmitted signals is identical,and the combined signal includes at least a combined information signal,the receiving communication device further comprising a signal detectorconfigured to detect the combined signal and determine the informationcontained in the combined information signal.
 27. The system of claim24, wherein the receiving communication device further comprises: adigital matched filter configured to generate a matched filter signalbased on the transmitted signals received by the receiving communicationdevice.
 28. The system of claim 24, wherein the receiving communicationdevice further comprises: a plurality of tapped delay lines configuredto modify the phase and amplitude of the transmitted signals.
 29. Thesystem of claim 24, wherein each of the transmitted signals includes aserial probe comprising a known data sequence, and wherein the receivingcommunication device determines a channel impulse response from theserial probe.
 30. The system of claim 24, wherein said receivingcommunication device heuristically combines the transmitted signals byphase matching and adding the magnitude of the transmitted signals. 31.The system of claim 24, wherein said receiving communication devicecomprises an equalizer.
 32. The system of claim 24, wherein at least oneof the transmitting communication devices is a mobile communicationdevice.
 33. The system of claim 24, wherein the receiving communicationdevice is a mobile communication device.
 34. A communication device fordetecting a plurality of signals respectively transmitted substantiallysimultaneously from a plurality of transmitting communication devicesover the same communication channel, the communication devicecomprising: a digital matched filter configured to generate a matchedfilter signal in response to reception of the transmitted signals at thecommunication device, wherein respective arrival times of thetransmitted signals are offset from one another as a function ofrespective positions of the transmitting communication devices; aplurality of tapped delay lines each configured to adjust a phase andfrequency of the matched filter signal in accordance with weightingcoefficients; a correlator configured to generate a correlation signalindicative of an amount of correlation among outputs of the plurality oftapped delay lines; a peak detector configured to detect peaks of thecorrelation signal; a phase rotator configured to rotate the phase ofdetected peaks of the correlation signal to account for timing offsetsamong the respective arrival times of the transmitted signals; acombiner configured to coherently combine the detected peaks to form acombined signal, thereby time aligning the transmitted signals tocompensate for the respective arrival times of the transmitted signals;and a detector configured to detect a presence of the transmittedsignals from the combined signal.
 35. The communication device of claim34, wherein in the plurality of tapped delay lines operates as anequalizer.
 36. The communication device of claim 34, wherein each of thetransmitted signals includes a serial probe comprising a known datasequence, and wherein said communication device determines a channelimpulse response from the serial probe and determines the weightingcoefficients from the channel impulse response.
 37. The communicationdevice of claim 34, said communication device heuristically combines thetransmitted signals by phase matching and adding the magnitude of thetransmitted signals.
 38. The communication device of claim 34, whereinthe communication device is a mobile communication device.
 39. Acommunication device for detecting a plurality of signals respectivelytransmitted substantially simultaneously from a plurality oftransmitting communication devices over the same communication channel,comprising: means for receiving the transmitted signals such thatrespective arrival times of the transmitted signals are offset from oneanother as a function of respective positions of the transmittingcommunication devices, each of the transmitted signals including aninformation signal which is coherently combinable with correspondinginformation signals in others of the transmitted signals; means for timealigning the transmitted signals to compensate for the respectivearrival times of the transmitted signals; means for combining thetransmitted signals to form a combined signal including at least acombined information signal; and means for detecting the combined signalto determine the presence of the transmitted signals.
 40. Thecommunication device of claim 39, wherein each of the transmittedsignals further comprises an acquisition signal, the communicationdevice further comprising: means for correlating the acquisition signalof transmitted signals received by the communication device to a storedsignal to estimate the arrival times of the transmitted signals.
 41. Thecommunication device of claim 39, wherein said means for time aligningincludes means for rotating a phase at least some of the transmittedsignals to adjust a relative timing of the transmitted signals toaccount for timing offsets among the respective arrival times of thetransmitted signals.
 42. The communication device of claim 39, whereinthe communication device is a mobile communication device.
 43. A networkof communication devices, comprising: a lead communication device and aplurality of other communication devices sharing a common time referencewith the lead communication device, wherein the lead communicationdevice commands the plurality of other communication devices to transmitsignals to a receiving device at a future time, and wherein the leadcommunication device and the plurality of other communication devicesrespectively transmit a plurality of signals at the future time, suchthat the plurality of signals are respectively transmitted over the samecommunication channel substantially simultaneously, each of theplurality of signals including an information signal which is coherentlycombinable with corresponding information signals in others of thesignals.
 44. The network of claim 43, wherein the lead communicationdevice broadcasts a command to the plurality of other communicationdevices.
 45. The network of claim 43, wherein a time between commandingof the plurality of other communication devices and the future time isgreater than a longest signal propagation time between the leadcommunication device and the other communication devices
 46. The networkof claim 43, wherein the common time reference is the time of day. 47.The network of claim 43, wherein the lead communication device is amobile communication device.
 48. The network of claim 43, wherein atleast one of the plurality of other communication devices is a mobilecommunication device.